**Exhibit 10.13**

Effective as of May 15, 2001

Ms. Tara Clark26 Hearthstone PlaceAndover, MA  01810

Dear Tara:

I am writing to confirm our offer to youfor a position as Vice President, Marketing, reporting to Walter Ogier,President and COO.  We are pleased tooffer you a starting salary of $10,903.75 per month (equivalent to $130,845.00per year), to be paid semi-monthly and reviewed annually.  The monthly salary includes an allowance tohelp offset contributions to certain benefits should you elect them.  You are also eligible for an annual cashbonus of 20% of base salary and to participate in our annual merit-based stockoption program, provided that any award of options will be subject to theapproval of our Board of Directors**.**You are also entitled to receive sharesof BTI common stock in accordance with the terms and conditions of the Eligix,Inc. Management Equity Incentive Plan. In addition, you will receive medicaland dental insurance, 15% of the cost of which is contributed by the employee.  Term life insurance, equivalent to one timesyour annual salary, and parking will be provided by the company withoutemployee contribution. Long-term disability insurance is provided, 100% of thecost is contributed by the employee (your annual premium would be approximately$767.00).  You will also be entitled tothree weeks paid vacation accrued on an annual basis and 11 paid holidays peryear.  In the event your employment isterminated by Biotransplant without cause, you would receive up to 6 monthsbase salary, payable in 12 equal semi-monthly installments which would bediscontinued upon your securing other employment.

Your employment is subject to yoursigning an Invention, Non-Disclosure and Non-Compete Agreement withBioTransplant, in the form previously provided to you, and review of anyagreement you may have with others to insure that your employment withBioTransplant is not in conflict with any such agreements.

Also, this offer is contingent upon yourability to provide proof of your legal right to work in the United States, asdefined by Federal regulations.

By signing this letter below, youacknowledge that the foregoing offered terms of your employment withBioTransplant do not constitute (i) a significant diminution of your duties,responsibilities, power, title or office in effect at Eligix, Inc. immediatelyprior to your acceptance of this offer or (ii) a material reduction in yourcompensation (including without limitation, salary, bonuses, options andbenefits) or material change in your manner of compensation (including withoutlimitation, the timing of any salary or bonus payments) in effect for you atEligix, Inc. immediately prior to your acceptance of this offer.

If you agree to the terms outlined above,please sign both copies of this letter and return one copy using the enclosed,stamped envelope.

We look forward to your joiningBioTransplant. We are confident that you will make a significant contributionto BioTransplant's future success.

Sincerely,

/s/ Elliot LebowitzElliot Lebowitz, Ph.D.President and CEO

Accepted and Agreed to:

| /s/  Tara Clark        |   |                     |
|------------------------|--------|--------------------------|
|                        |   |                     |
| Name:  Tara Clark |   | Date:  May 15, 2001 |